1. Field of the Invention
The present invention relates to earth-boring drill bits of the type which include exceptionally deep junk slots.
2. Description of the Related Art
Typically, earth boring drill bits include a bit body which may be of steel or may be fabricated of a hard matrix material such as tungsten carbide. A plurality of diamond or other cutting elements are mounted along the exterior face of the bit body. Each diamond cutting element typically has a backing portion which is mounted in a recess in the exterior face of the bit body. Depending upon the design of the bit body and the type of diamonds used (i.e., either natural or synthetic), the cutters are either positioned in a mold prior to formation of the bit body or are secured to the bit body after fabrication.
The cutting elements are positioned along the leading edges of the bit body so that as the bit body is rotated in its intended direction of use, the cutting elements engage and drill the earth formation. In use, tremendous forces are exerted on the cutting elements, particularly in the forward to rear tangential direction as the bit rotates, and in the axial direction of the bit. Additionally, the bit body and cutting elements are subjected to substantial abrasive and erosive forces.
Tungsten carbide or other hard metal matrix bits have the advantage of high erosion and abrasion resistance. The matrix bit is generally formed by packing a graphite mold with tungsten carbide powder and then infiltrating the powder with a molten copper alloy binder. A steel blank is positioned in the mold and becomes secured to the matrix as the bit cools after furnacing. Also present in the mold is a mandrel which, when removed after furnacing, leaves behind the fluid passages through the bit. After molding and furnacing of the bit, the end of the steel blank can be welded or otherwise secured to an upper threaded body portion of the bit.
Such tungsten carbide or other hard metal matrix bits, however, are brittle and can crack upon being subjected to impact forces encountered during drilling. Additionally, thermal stresses from the heat applied during fabrication of the bit or during drilling may cause cracks to form. Finally, tungsten carbide and other erosion resistant materials are very expensive in comparison with steel as a material of fabrication.
U.S. Pat. No. 4,884,477 to Smith et al discloses a method for making a drill bit in which relatively ductile filler material is placed in the bit mold prior to infiltrating the same in a furnace. A hard metal matrix material is provided on the exterior surface of the crown and gauge of the bit while increasing the overall ductility of the matrix thus providing a less brittle and yet still wear resistance bit.
For many years, diamond bits which incorporated a bit blank in an infiltrated matrix were shaped much like the bit disclosed in the Smith patent. The matrix comprises a relatively thin layer of hard matrix comprising the gage and crown of the bit. Junk slots, which comprise grooves formed in the matrix via junk slot displacements in the mold, extend from the crown vertically up the sides of the gauge. The junk slots provide a flow path for drilling fluid which circulates out of the lowermost portion of the bit into the borehole to cool the bit with less cuttings from the bore.
In some soft formation bits constructed as described above, drilling fluid cannot flow at sufficiently high rates to circulate the cuttings from the well bore due to the relatively small cross sectional flow area between the bit crown and the borehole, which restricts fluid flow. Although such bits include junk slots as described above, they are not sufficiently deep or wide enough to accommodate a sufficiently high flow rate.